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To realize the sustainable development of water ecology in watersheds, we use the emergy triangle to infer the degree of environmental sustainability. R represents renewable resources, N means non-renewable resources and F represents the economic input of environmentally friendly production. ESI is the environmental sustainability index, and ESI = [(R + N + F)/F]/[(N + F)/R]. In short, the inputs of ecosystems are classified into three types: renewable resources in the watershed (R), nonrenewable resources in the watershed (N) and economic inputs of environmentally friendly production (F). F is provided by the market or economic flows. In Figure 4, the sustainability lines depart from the apex N which leads to the R and F lines allowing the division of the triangle into sustainability areas. This triangle is useful for the identification and comparison of the sustainability of products and processes. The upper part of the diagram represents the regions (ESI > 5) where systems are sustainable, the middle part represents the regions (1 < ESI < 5) in which the systems are sustainable for the medium term, and the lower part of the diagram represents the regions (ESI < 1) where systems are not sustainable (Almeida et al. 2007). The indicator sometimes neglects the role of local non-renewable resources (N). ESI takes both ecological and economic compatibility into account, which evaluates the sustainability of a process or system. The larger the ESI, the higher the sustainability of a system is. The parameters used in emergy calculation are provided in Table 2. For ESI, it would be more sustainable to exploit all the non-renewables in an area (for N → 1, ESI → R/F) than to have a relatively important but low amount of exogenous resources (F), so the indicator sometimes neglects the role of local non-renewable resources (N). However, depending on the viewpoint of efficiency, renewability and external inputs, ESI is more efficient (lower transformity) in the case of relatively lower non-renewable inputs (lower environmental loading ratio (ELR)) and higher emergy yield ratio (EYR).
Table 2

Emergy calculation indices

SymbolsDescriptionsEquations
EYR EYR: the ratio of the emergy of the output (Y = R + N + F) to the emergy of input (F).  
ELR ELR: the ratio of non-renewable emergy (N + F) to renewable emergy (R). It is an indicator of the pressure on the ecosystem due to production activity.  
ESI ESI: to obtain the highest yield ratio at the lowest environmental loading.  
SymbolsDescriptionsEquations
EYR EYR: the ratio of the emergy of the output (Y = R + N + F) to the emergy of input (F).  
ELR ELR: the ratio of non-renewable emergy (N + F) to renewable emergy (R). It is an indicator of the pressure on the ecosystem due to production activity.  
ESI ESI: to obtain the highest yield ratio at the lowest environmental loading.  
Figure 4

A schematic diagram of the relationship between ESI and the implementation of eco-compensation.

Figure 4

A schematic diagram of the relationship between ESI and the implementation of eco-compensation.

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